11June 2021 ESRFnews

INSIGHT

between emittance and dynamic aperture, giving excellent values for both parameters. In the case of PETRA, the H6BA version could deliver 10 pmrad emittances and beam lifetimes in excess of 100 hours. Raimondi remarked that this design, which is a direct evolution of the HMBA optimised to the PETRA site boundary conditions, extremely efficiently implements the delivery of a diffraction-limited cylindrical beam with fewer components than the present PETRA III storage ring; it also has a simpler assembly, 70% power reduction per unit length and as a crude estimate 70% of the EBS upgrade cost for a single cell (although the H6BA PETRA lattice has 72 cells compared with the ESRF EBS s 32). Finally, because all the upgrade tools and expertise have already been developed at the ESRF, PETRA can be upgraded very rapidly and cost-effectively. It is very fortunate that the best moment for a facility to start building a new accelerator is the day after another one has been successfully built, Raimondi said. Not only does this show how influential the EBS design could be, but it also shows the importance of the return provided by the international organisation that is the ESRF to the synchrotron-radiation community at large, and in particular to the national sources of its member states.

Jon Cartwright

sections, the injector complex and the 844-metre circumference footprint essentially unchanged. As Raimondi pointed out in his webinar, existing synchrotron facilities or greenfield sites with much larger footprints ought to be able to reduce the emittance further down to 10 pmrad or less using his HMBA concept and latest accelerator technology, with little difficulty. The challenge, as with the EBS, he said, will be coping and working within many other limitations required for optimal beamline-operation conditions and site constraints.

Such as? It depends on the site, and the synchrotron. For instance, if the HMBA concept were transferred directly to PETRA at DESY in Germany a synchrotron with a circumference of 2.3 km that is planning a fourth- generation upgrade it would result in a horizontal emittance in the region of 17 pmrad, but also a beam lifetime of less than half an hour. Pondering this has led Raimondi to develop yet another type of cell, the hybrid six-bend achromat (H6BA), in which among other changes the first dipole is scrapped and a quadrupole focusing magnet is installed in the middle.

What could be the benefits of the H6BA? According to Raimondi, the HMBA bucks the trend of previous cell designs, which had to strike a balance

A revolution in itself, the HMBA lattice is inspiring even more brilliant synchrotron light sources.

The ESRF EBS has already opened a new vista for X-ray science, hasn t it? Yes. Last year, following an intense upgrade project lasting five years, the ESRF re-opened to users as the ESRF EBS, the brightest and most coherent fourth-generation light source. A major goal of the upgrade was to provide ESRF users with a tool to probe everyday objects and the living world with unparalleled depth, speed and resolution indeed, the tool is so advanced that it is expected to be years before users learn to take full advantage of it. However, a secondary goal of the EBS upgrade was always to further accelerator science and engineering for the benefit of the broader light- source community. Since its inception more than two decades ago, the ESRF has served as a reference model for the construction of other first-class national and international facilities. Now, as explained in an online webinar in April by Pantaleo Raimondi, the former director of the ESRF s accelerator and source division, the ESRF EBS is following suit, inspiring the construction of many other fourth- generation synchrotrons in Europe, the US, Japan, Russia and China.

In what way? Improving the X-ray coherence of the ESRF meant significantly lowering the horizontal spread, or emittance, of the orbiting electrons in its source. To do this, Raimondi invented the hybrid multi-bend achromat (HMBA), a highly tuned bending cell based on multiple dipole and dipole-quadrupole magnets, along with dozens of other magnets to impose subtle beam corrections. The design worked in its H7BA EBS version, delivering electron beams with a horizontal emittance of just 133 picometre milliradians (pmrad), a 100- fold improvement. But more amazing was that it worked given the ESRF s various constraints and requirements: a reduction in operating costs and electricity consumption, while leaving the existing beamlines, the straight

Fresh after its upgrade, the ESRF is showing where synchrotrons can go next.

Beyond the EBS

S TE

F C A N D É /E S R F

The ESRF has long served as a reference model for other national and international facilities